1. Field of the Invention
The present invention relates to a semiconductor memory device. Further, the present invention relates to a semiconductor device including the semiconductor memory device.
2. Description of the Related Art
In recent years, most electronic devices such as computers can perform desired operations by using a variety of data. When the data is held in, for example, a memory device (also referred to as a memory) or the like, the data can be used temporarily or semipermanently. A memory device also includes an external memory device (an auxiliary memory device) such as a hard disk or a flexible disk in a broad sense. However, a memory device almost always refers to a semiconductor memory device such as a CPU (central processing unit). Two main types of memory devices are a volatile memory and a nonvolatile memory. A volatile memory refers to a memory device in which data is lost over time after the data is held. A nonvolatile memory refers to a memory device in which data can be held semipermanently after the data is held.
Although a volatile memory has a possibility of losing data, it has an advantage of short access time. On the other hand, although a nonvolatile memory can hold data semipermanently, it has a disadvantage of high power consumption. Memory devices each have features in this manner, and each of the memory devices is used in accordance with the kind or usage of data.
Among the memory devices, there is a memory device functioning as a memory device (hereinafter referred to as a resistive random access memory device) by which the resistance of a memory element is changed, voltages before and after change in the resistance is determined, and the voltages are held as data. An example of a resistive random access memory device is described below. The resistive random access memory device includes a memory element having a structure in which a layer (hereinafter also referred to as a compound layer) which includes at least a layer of an organic compound or an inorganic compound such as silicon is interposed between a pair of electrodes. The memory element applies high voltage between a pair of electrodes, and the electrodes are short-circuited. Resistance of the memory element decreases according to this. In the above described resistive random access memory device, writing state or non-writing state is determined by existence of the short-circuit in the memory element, and voltage of each state is held to serve as a memory device (hereinafter referred to as a short-circuit type memory). (For example, Patent Document 1: Japanese Published Patent Application No. 2007-012035)
A conventional memory device shown in Patent Document 1 has a structure including a memory circuit provided with a select transistor connected to a word line and a bit line, and a memory element connected to one of the source and the drain. In a memory circuit electrically connected to a word line and a bit line, predetermined levels of potential are applied to a first terminal and a second terminal of the memory element through the word line and the bit line. The memory element is short-circuited when the potential difference between the first terminal and the second terminal is higher than or equal to a predetermined value (data writing state), and the memory element is not short-circuited when the potential difference is lower than a predetermined value (data non-writing state). Thus, data of 1 or 0 is stored in each memory element. Further, since a select transistor selects whether data is written in the memory element or not according to the potential from the outside, a select transistor with little variation is preferable. For example, a transistor of which a gate insulating film is thinned is preferred. This is because a gate insulating film is thinned, so that variation can be reduced, and capacitance per unit area can be increased.